Various types of rotary drill bits or rock bits may be used to form a bore hole in the earth. Examples of such rock bits include roller cone bits or rotary cone bits used in drilling oil and gas wells. A typical roller cone bit comprises a bit body with an upper end adapted for connection to a drill string. A plurality of support arms, typically three, depend from the lower end portion of the bit body with each arm having a journal protruding radially inward and downward with respect to a projected rotational axis of the bit body.
Conventional roller cone bits are typically constructed in three segments. The segments may be positioned together longitudinally with a welding groove between each segment. The segments may then be welded with each other using conventional techniques to form the bit body. Each segment also includes an associated support arm extending from the bit body. An enlarged cavity or passageway is typically formed in the bit body to receive drilling fluids from the drill string. U.S. Pat. No. 4,054,772 entitled, Positioning System for Rock Bit Welding shows a method and apparatus for constructing a three cone rotary rock bit from three individual segments. U.S. Pat. No. 4,054,772 is incorporated by reference for all purposes within this application.
A cutter cone is generally mounted on each journal and supported rotatably on bearings acting between the journal and the inside of a journal receiving cavity in the cutter cone. Typically, the bearings are spaced from the journal by a diametral distance on the order of 0.007 inches. In a sealed roller cone bit, a lubricant under pressure is forced into the space between the journal and the bearings. A lubricant reservoir is used to compensate for any partial grease loss and to balance the internal lubricant pressure to the external hydrostatic pressure during the drilling operation. The lubricant may comprise, for example, a calcium complex grease. Additionally, solids, such as molybdenum disulfide, may be added to the lubricant to increase the load-carrying capacity of the bearings.
The bearings in a roller cone rock bit are heavily loaded during downhole drilling operations. In such drilling operations, the roller cone rock bit is rotated in a bore hole which causes the cutter cones to rotate on the journals. The roller cone rock bit typically operates at a low speed with heavy weight applied to the bit which also produces a high load on the associated bearings. This makes it difficult to generate a lubricant film between the surfaces of the bearing and the journal. If the lubricant film is too thin, the surfaces may come in contact causing additional friction and heat which can ultimately cause failure of the drill bit. For example, the drill bit may comprise one or more elastomeric seals which may be degraded by exposure to high temperatures created by excessive friction. This may allow water, mud, and other debris from the drilling operation to penetrate the space between the roller cone and the journal and thus increase the wear on the bearing surfaces to the point where the roller cones are lost in the hole.
Some prior bearings have been designed to compensate for the difficulty in generating a sufficient lubricant film. For example, U.S. Pat. No. 4,787,129 entitled METAL OF MANUFACTURING A COMPOSITE JOURNAL BUSHING discloses a bearing that incorporates a thin layer of a metal such as silver. The silver layer on the bearing surface acts as a solid lubricant. However, the addition of this layer is difficult and expensive.